Large scale solar power plants utilizing concentrating solar power (“CSP”) technology have been producing power for over thirty years. The Solar Electric Generating Systems (“SEGS”) facilities in the Mojave Desert of California are a well-known example of solar power plants using such CSP technology. Other types of solar thermal power plants are in operation in various other areas of the World. CSP utilizes solar collectors comprising large mirrors, mirror arrays, or lenses, which concentrate solar energy upon a typically unpressurized pipe or tube that contains a heat transfer fluid. Synthetic or organic oils having a high boiling point or salts are used as the heat transfer fluid in a variety of power plant configurations. As an example, some of the SEGS facilities utilize Therminol® from Solutia, Inc. as the heat transfer fluid.
As the heat transfer fluid flows through the unpressurized pipe inside the solar collectors, the transfer fluid is heated by the incident sunlight. One or more pumps are situated along the pipe to pump the fluid through the solar collectors and towards a boiler with a heat exchanger coil. At the heat exchanger coil, the transfer fluid is used to heat water in the boiler to produce steam. The steam is then used for powering a steam driven engine that turns a generator to produce electricity. After the heat transfer fluid releases its thermal energy in the boiler, the heat transfer fluid is pumped back to the solar collectors to be heated again and the closed cycle continues.
A disadvantage of the use of oils as heat transfer fluids is that the fluid has a relatively low energy density. For example, Therminol® has an energy density of approximately 2100 joules per kilogram degree Celsius (J/kg° C.) whereas ordinary water has an energy density of approximately 4200 J/kg° C. This relatively low energy density for Therminol® means that it carries less thermal energy from the solar collectors to the heat exchanger coil than water thus resulting in a larger and more costly required set of heat transfer components.
Another disadvantage of synthetic heat transfer fluids is that they are often flammable. A fire at one of the SEGS plants could cause massive damage and could result in personally injury or death to power plant workers. As a result, care must be taken in handling the fluids to keep the fluids from overheating.
For these and other reasons, a number of solar power systems have been developed to produce steam directly from water rather than using a synthetic heat transfer fluid. Such systems—dubbed Direct Solar Steam generation (“MS”) or Direct Steam Generation (“DSC”)—distribute water through the unpressurized pipes in the solar collectors rather than distributing a synthetic heat transfer fluid. Because water has a much lower boiling point than a synthetic heat transfer fluid, the water will eventually turn to steam after being heated a sufficient amount. Thereafter, the steam is directed to a steam turbine for generating electricity.
Such DSG systems have their own drawbacks. The presence of steam in the pipes of the solar receivers reduces the efficiency of the collectors and receivers because steam has a significantly lower capacity to absorb heat than liquid water. Thus, the steam can carry less thermal energy towards the turbine than can pressurized water. Further, the use of a two-phase (water/steam) flow within the pipes of long linear solar receivers creates a condition known as the Ledinegg Instability. This phenomenon results in a boiling front as the water moves through the pipes and flashes over to steam. To compensate for this instability, an undesirable pressure drop must be introduced into the system. Finally, DSG systems are more sensitive to variations in solar flux density and changes in atmospheric conditions because the systems will not function properly unless the water in the solar collectors is sufficiently heated to flash over to steam at a required rate. Taken together, these drawbacks necessitate the use of larger, more expensive solar collectors to produce a required amount of steam to produce electricity. Therefore, such DSG systems may have little or no cost savings in comparison to traditional CSP systems containing synthetic heat transfer fluid.